The joining of metals such as aluminum alloys or copper alloys is complicated by the presence of rapidly forming oxide layers as well as their inherently high electrical and thermal conductivity. Spot welding can be difficult and requires preparation of the workpiece surface, high currents, high forces, and mechanically well-aligned and stable electrodes. Short electrode life is common because of reaction between the aluminum workpiece alloy and the copper electrode or welding of the copper workpiece alloy to the copper electrode. The resultant weld between aluminum workpieces can have high strength, but poor high cycle fatigue performance under some conditions.
Lithium batteries for vehicle applications require a process to join the battery cells to a conductor or bus bar. The battery cells typically use thin aluminum and copper sheets as electrode substrates. These electrode sheets incorporate an extension, known as a tab, which extends outside of the cell pouch and is used to join the electrode sheet to the copper conductors or bus bars during battery assembly. Two types of tab materials are commonly used in battery construction: aluminum and copper. In some cases, the copper tabs and/or copper conductor may be coated with a thin layer of nickel to enhance corrosion resistance and joining while aluminum tabs are coated with an anodization layer.
Joining the thin tab materials to the much thicker copper conductor is difficult for several reasons. First, the stack-ups require joining several separate pieces of metal in one operation, e.g., three separate tabs to one conductor. Second, one of the stack-ups includes a metal combination that is known to form brittle intermetallics, i.e., copper and aluminum. Third, the thickness ratio between the conductor and tabs is high, typically at least about 5:1.
Ultrasonic welding has been used for this application with some success. It enables the joining of dissimilar metals and is capable of joining materials with significant differences in sheet thickness. However, there is considerable difficulty in joining stack-ups that contain more than two sheets because the ultrasonic energy, i.e., vibrations parallel to the sheet surface, does not transfer well across the sheet-to-sheet interfaces. The top sheet couples well to the ultrasonic energy source because it is in direct contact with the ultrasonic tool or sonotrode. Hence it reacts strongly with the adjacent sheet. However, sheets located lower in the stack, including the conductor bar, do not receive as much ultrasonic energy, and the resulting weld may not be as strong.
Mechanical fasteners such as screws or clamps have also been used. They rely on very low contact resistance to achieve good electrical conductivity. However, contact resistance can degrade over time through the build-up of surface contaminants, e.g., oxides, or through degradation of the fastener.
Soldered joints can also be used. However, the use of solders with fluxing agents, particularly for aluminum, can result in the formation of corrosive flux residue that will degrade the surrounding materials or joint over time if not removed by cleaning operations. These operations add cost and, in some cases, may not be possible depending on the assembly sequence.
There remains a need for a process for welding battery cell tabs to conductors or bus bars.